Upwelling Of Water Research Articles

Induced polarization images alteration in stratovolcanoes

We investigate the complex conductivity of 32 volcanic rock samples from two stratovolcanoes, La Soufrière Volcano (Guadeloupe Island, Caribbean) and Papandayan volcano (Java Island, Indonesia). These stratovolcanoes are characterized by high degrees of kaolinite-related alteration associated with the upwelling of acidic ground waters as well as the formation of smectite-rich clay caps. Our goal is to assess the dependence of two geoelectrical properties, the electrical conductivity and normalized chargeability, on the conductivity of the pore water and the Cation Exchange Capacity (CEC) for volcanic rocks at near-neutral pH conditions. An alteration index based on CEC is built for both smectite- and kaolinite-rich zones. The data are discussed in the context of a previously acquired experimental dataset based on samples collected from shield volcanoes in Hawaii. We show that all the core samples display the same trends in their petrophysical geoelectrical properties whatever the type of volcanoes. Surface conductivity and normalized chargeability are strongly controlled by the CEC of the material and the bulk tortuosity of the pore space (product of the formation factor by the porosity). This implies in turn that the conductivity and the normalized chargeability are controlled by rock alteration in the same way as long as surface conductivity dominates the bulk conductivity contribution, which is salinity and pH dependent (both being interrelated in acidic pore waters). These petrophysical results are then applied to the interpretation of a 3D induced polarization survey performed at Papandayan stratovolcano in Indonesia. The tomograms are used to image the alteration of the volcanic edifice and we conclude that surface conduction is non-negligible in these volcanoes. Only above a pore water conductivity higher than 10 S m−1 at 25 °C) the bulk conductivity dominates the surface conductivity associated with alteration.

Spatio-Temporal Distribution of Juvenile Oceanic Whitetip Shark Incidental Catch in the Western Indian Ocean

Oceanic whitetip shark (Carcharhinus longimanus) is an important top predator in pelagic ecosystems currently classified as globally Critically Endangered by the International Union for the Conservation of Nature. This species is incidentally caught by fisheries targeting highly migratory tunas and billfishes throughout the Indian Ocean. Understanding the temporal, spatial and environmental factors influencing the capture of this species is essential to reduce incidental catches. In this study, we used generalized additive models to analyze the spatio-temporal distributions of the juvenile oceanic whitetip shark catches and the environmental conditions in the western Indian Ocean using observer data from 2010 to 2020 of the European Union and associated flags purse seine fishery. We found sea surface temperature and nitrate concentration to be the most important environmental variables predicting the probability of catching an oceanic whitetip shark. A higher probability of capture was predicted in areas where sea surface temperature was below 24°C and with low nitrate concentrations close to zero and intermediate values (1.5-2.5 mmol.m-3). We also found a higher probability of capture in sets on fish aggregating devices than in sets on free schools of tuna. The Kenya and Somalia basin was identified to have higher probabilities of capture during the summer monsoon (June to September) when upwelling of deep cold waters occurs. We provide the first prediction maps of capture probabilities and insights into the environmental preferences of oceanic whitetip shark in the western Indian Ocean. However, the causal mechanisms behind these insights should be explored in future studies before they can be used to design spatial management and conservation strategies, such as time-area closures, for bycatch avoidance.

Dynamics of a Standing Meander of the Subantarctic Front Diagnosed from Satellite Altimetry and Along-Stream Anomalies of Temperature and Salinity

Abstract Meanders formed where the Antarctic Circumpolar Current (ACC) interacts with topography have been identified as dynamical hot spots, characterized by enhanced eddy energy, momentum transfer, and cross-front exchange. However, few studies have used observations to diagnose the dynamics of ACC standing meanders. We use a synoptic hydrographic survey and satellite altimetry to explore the momentum and vorticity balance of a Subantarctic Front standing meander, downstream of the Southeast Indian Ridge. Along-stream anomalies of temperature in the upper ocean (150–600 m) show along-stream cooling entering the surface trough and along-stream warming entering the surface crest, while warming is observed from trough to crest in the deeper ocean (600–1500 m). Advection of relative vorticity is balanced by vortex stretching, as found in model studies of meandering currents. Meander curvature is sufficiently large that the flow is in gradient wind balance, resulting in ageostrophic horizontal divergence. This drives downwelling of cooler water along isopycnals entering the surface trough and upwelling of warmer water entering the surface crest, consistent with the observed evolution of temperature anomalies in the upper ocean. Progressive along-stream warming observed between 600 and 1500 m likely reflects cyclogenesis in the deep ocean. Vortex stretching couples the upper and lower water column, producing a low pressure at depth between surface trough and crest and cyclonic flow that carries cold water equatorward in the surface trough and warm water poleward in the surface crest (poleward heat flux). The results highlight gradient–wind balance and cyclogenesis as central to dynamics of standing meanders and their critical role in the ACC momentum and vorticity balance. Significance Statement The Antarctic Circumpolar Current (ACC) in the Southern Ocean is a nearly zonal current that encircles Antarctica. It acts as a barrier between warmer water equatorward and colder water poleward. In a few regions where the current encounters strong topographic changes, the current meanders and opens a pathway for heat to travel across the ACC toward Antarctica. We surveyed a meander in the ACC and examined the along-stream change of temperature. In the upper ocean, temperature changes are caused by a vertical circulation, bringing cool water down when entering the surface trough (the part of the meander closest to the equator), and warm water up when exiting the surface trough and entering the surface crest. At depth, cold water is transported equatorward in the surface trough and warm water poleward in the surface crest, leading to a net transport of heat poleward. This study highlights the importance of the secondary circulation within a meander for generating cross-ACC flows and moving heat toward Antarctica.

Wind‐Induced Hypolimnetic Upwelling Between the Multi‐Depth Basins of Lake Geneva During Winter: An Overlooked Deepwater Renewal Mechanism?

AbstractCombining field observations, 3D hydrodynamic modeling, and particle tracking, we investigated wind‐induced interbasin exchange between the Petit Lac (PL) (depth 75 m) and Grand Lac (GL) (depth 309 m) basins of Lake Geneva in early winter. Following a strong 2.5‐day wind event, a two‐layer flow field developed, where the downwind surface drift into the GL was balanced by counterflowing hypolimnetic currents into the PL. Velocities in both layers exceeded 20 cm s−1, with the highest values (27 cm s−1) found near the bottom. For 3.5 days, hypolimnetic temperatures at the confluence decreased to values found in the deep GL hypolimnion at 180‐m depth. Approximately 1.5 days after the wind event ceased, currents reversed and upwelled waters returned into the deep GL hypolimnion. The Coriolis force strongly modified the interbasin exchange dynamics, which were well represented by the model. Particle tracking revealed a “current loop,” that is, water from below 150‐m depth first upwelled into the PL, intruded approximately 10 km (half its length), and then descended back into the GL hypolimnion. Model results showed that the PL hypolimnetic volume doubled during the upwelling. Low model‐based gradient Richardson numbers and temperature inversions in CTD profiles indicated turbulent mixing between the upwelled GL and surrounding PL waters. Our findings demonstrate that hypolimnetic upwelling between the two basins frequently occurs during winter and could potentially be an important, but as yet overlooked mechanism for hypolimnetic‐epilimnetic exchange and deepwater renewal in Lake Geneva, and probably in other multi‐depth basin lakes under similar wind conditions.

Physical Drivers of Biogeochemical Variability in the Polar Front Meander

AbstractThe Southern Ocean plays a vital role in global ocean circulation, and the Polar Front (PF) is one of its most important physical features. The PF meander south of Tasmania, around 153°E, 55°S, is a very dynamic region which spawns mesoscale eddies, and influences local biogeochemistry and sea‐air interaction. By using voyage and ancillary data, we investigated the unusually strong spring bloom in the vicinity of the PF meander in 2018. We infer that the upwelling of deep water at the front and in eddies, brings macronutrients and dissolved iron (dFe) to the surface. Chlorophyll concentration peaked at over 0.6 mg m−3, which is anomalously high for this area. With reduced iron limitation, the physiological characteristics of phytoplankton in the northern, downstream part of the study area also changed. The photochemical efficiency was improved and released this area from its usual high‐nutrient low‐chlorophyll (HNLC) status. This was mainly indicated by the increase in the dawn Fv/Fm maximum (indictor of photochemical efficiency) from 0.2 to over 0.5. With the biomass increase and healthier community status, we observed consumption of surface dissolved inorganic carbon and increased particulate organic carbon production to about 40 μmol L−1, forming a weak local carbon sink. Through the investigation of multiple years, a weak positive correlation between mixed layer depth shoaling and phytoplankton growth was found, but there was significant interannual variability in this relationship, likely caused by variable eddy conditions and dFe delivery.

Top-Down and Bottom-Up Control in the Galápagos Upwelling System

Increased standing macroalgal biomass in upwelling zones is generally assumed to be the result of higher nutrient flux due to upwelled waters. However, other factors can strongly impact macroalgal communities. For example, herbivory and temperature, via their effects on primary producers and the metabolic demands of consumers, can also influence macroalgal biomass and productivity, respectively. We assessed the effects of nutrient availability, temperature, and herbivory on macroalgal biomass on a subtidal nearshore rocky reef in the Galápagos Islands. We manipulated nutrient availability and herbivory in field experiments performed in two seasons: the first during a cool, upwelling season, and the second during a warm, non-upwelling season. Excluding macro-herbivores had a clear effect on standing macroalgal biomass, independent of season or nutrient availability. However, we found different interactive effects of nutrients and macro-herbivores between the two seasons. During the cool season, macroalgal biomass was significantly higher in herbivore exclusions than in open areas under ambient nutrient conditions. However, when nutrients were added, macroalgal biomass was not significantly different across all herbivore treatments, which suggests reduced top-down control of herbivores (hence a greater standing algal biomass) in open areas. In the warm season, macroalgal biomass was significantly higher in herbivore exclusions compared to open treatments, both with and without nutrient addition. Furthermore, biomass reached 11X in herbivore exclusions with nutrient additions, which hints nutrient limitation only during warm, low-upwelling conditions. Overall, our results support the hypothesis that macro-herbivores reduce macroalgal biomass in this system and suggest that nutrient availability, but not temperature, modulate herbivory.

N2O dynamics in the hyporheic zone due to ripple migration

The migration rate of ripples along the streambed has a profound effect on the hyporheic exchange and nitrogen dynamics. However, how the denitrification intermediate N2O is released in mobile bedforms still remain unclear. In this study, the ripple domains are established based on the sediment stability criteria by using the mean channel flow velocity U and the median sediment grain size D50. A two-dimensional numerical model coupling hyporheic flow, ripple migration and N2O transformation is proposed to investigate how ripple celerity affects N2O dynamics in streambeds. The results show that there are different patterns for the release of N2O from migrating ripples with different medium grain sizes D50 (0.06–0.3 mm). N2O can be released via upwelling of anoxic pore water from the deeper area in immobile or slow-mobile ripples. However, rapid ripple migration and the resulting fast vertical flow reversal can suppress the net pore water displacement and lead to the formation of a redox seal, which separates the deeper anoxic area from the above oxic area and inhibits N2O emissions. The magnitude of the nutrients concentration gradient and the N2O flux between these two areas depend only on the dispersive and diffusive transport. Therefore, the controls of N2O emission flux hence shift from the pumping- dominated regime in stable ripples to a grain size dominated regime of the migrating ripples. The possible implications about the effects of hydrodynamic changes due to the migration of ripples are presented. Our results emphasize that knowledge of N2O production-consumption hotspots within streambeds is essential to understand the mechanisms of N2O emissions from the sediment–water interface.

Satellite observations estimating the effects of river discharge and wind-driven upwelling on phytoplankton dynamics in the Chesapeake Bay.

Phytoplankton growth in estuaries is regulated by a complex combination of physical factors with freshwater discharge usually playing a dominating role controlling nutrient and light availability. The role of other factors, including upwelling-generating winds, is still unclear because most estuaries are too small for upwelling to emerge. In this study, we used remotely sensed proxies of phytoplankton biomass and concentration of suspended mineral particles to compare the effect of river discharge with the effect of upwelling events associated with persistent along-channel southerly winds in the Chesapeake Bay, a large estuary where upwelling and its effects on biogeochemical dynamics have been previously reported. The surface chlorophyll-a concentrations (Chl-a) were estimated from Visible Infrared Imaging Radiometer Suite (VIIRS) satellite data using the Generalized Stacked-Constraints Model (GSCM) corrected for seasonal effects by comparing remotely sensed and field-measured data. Light limitation of phytoplankton growth was assessed from the concentration of suspended mineral particles estimated from the remotely sensed backscattering at blue (443 nm) wavelength bbp (443). The nine-year time series (2012-2020) of Chl-a and bbp (443) confirmed that a primary factor regulating phytoplankton growth in this nearshore eutrophic area is discharge from the Susquehanna River, and presumably the nutrients it delivers, with a time lag up to four months. Persistent southerly wind events (2-3 days with wind speed >4 m/s) affected the water column stratification in the central part of the bay but did not result in significant increases in remotely sensed Chl-a. Analysis of model simulations of selected upwelling-favorable wind events revealed that strong southerly winds resulted in well-defined lateral (East-West) responses but were insufficient to deliver high-nutrient water to the surface layer to support phytoplankton bloom. We conclude that, in the Chesapeake Bay, which is a large, eutrophic estuary, wind-driven upwelling of deep water plays a limited role in driving phytoplankton growth under most conditions compared with river discharge. Integr Environ Assess Manag 2022;18:921-938. © 2022 SETAC.

Progressive Intensification of Pacific Deep Water Circulation Since the Early Pliocene

AbstractAn important stage in global ocean overturning circulation is the upwelling of the Antarctic Bottom Water into the Pacific Deep Water (PDW). The long‐term evolution of PDW circulation, however, is scarcely addressed. We use seismic reflection data and cores from the International Ocean Discovery Program (and its predecessors) in contourites to reconstruct regional deep ocean circulation patterns. Here, we demonstrate that PDW circulation was progressively enhanced at about 4.35, 3.84, 2.16, and 0.97 Ma since the early Pliocene. Coeval growth of volcanic arcs and tectonic uplifts gradually isolated marginal seas from the PDW and, in conjunction with climate cooling, effectively increasing deep‐water flux within a shrinking Pacific basin. Regional stratigraphic discontinuities in contourites indicate thresholds in the intensification of ocean circulation.

Interactions Between Ocean and Successive Typhoons in the Kuroshio Region in 2018 in Atmosphere–Ocean Coupled Model Simulations

AbstractTyphoons decrease sea surface temperature (SST) along their wakes through upwelling of subsurface water, vertical mixing in the upper ocean, and heat release from the sea surface, and these cold wakes can influence subsequent typhoons. In this study, we investigated interactions between the upper ocean and typhoons in the North Pacific subtropical gyre with focuses on Kuroshio's response and feedback using atmosphere–ocean coupled model simulations. In late August and early September 2018, typhoons SOULIK, CIMARON, and JEBI passed through the northwestern subtropical gyre. During the passages of SOULIK and CIMARON, SST decreased along their paths due to vertical mixing except in the Kuroshio region. We quantitatively revealed that the Kuroshio stayed warm because the deep mixed layer along its path and small vertical temperature gradient around the mixed layer base, which are unfavorable conditions for cooling by vertical mixing, limited the cooling effects. After SOULIK and CIMARON had passed, SST recovered through horizontal Kuroshio heat transport and radiative heating. The possibility that the SST field after SOULIK and CIMARON passages influences JEBI was also discussed. Although impacts on JEBI intensity were not identified, it is implied that the turbulent heat flux (THF; sum of the sensible and latent heat fluxes) around JEBI was modulated by the SST field: heat release from the ocean was reduced in the region with decreased SST and enhanced over the sustained high SST of the Kuroshio. Furthermore, the large THF over the Kuroshio may have caused an increase of JEBI‐associated precipitation around Japan.

Coupled nutricline and productivity variations during the Pliocene in the western Pacific warm pool and their paleoceanographic implications

The tropical Pacific played an important role in modulating global climate change during the Pliocene. Studies of tropical Pacific sea surface temperatures covering the period from the Pliocene onwards indicate that changes in the thermal mean state over the tropical Pacific can significantly influence global climate feedbacks and connect the high- and low-latitude climates. Tropical productivity fluctuations are a significant mechanism with respect to the operation of the global carbon cycle. Yet, temporal changes in primary productivity are not well constrained in the western Pacific warm pool (WPWP), where the ocean–climate system is not dominated by upwelling systems. Furthermore, the role of nutricline dynamics in forcing productivity over tectonic timescales remains uncertain. Here we use relatively high-resolution foraminiferal carbon isotope records combined with Ba/Ti ratios obtained from International Ocean Discovery Program (IODP) Site U1490 in the WPWP to reconstruct nutricline depth and paleoproductivity over the period 5.1–2.6 Ma. Our records imply that nutricline and productivity variations were closely coupled over tectonic timescales, implying that the dynamics of the nutricline play a significant role in regulating productivity in the WPWP. The deeper nutricline and lower productivity during 4.8–3.5 Ma might have been fostered by the closure of the Central American Seaway through the thickening of the mixed layer in the WPWP. We relate the overall shallower nutricline and increased productivity during 3.5–3.0 Ma to the restriction of the Indonesian Seaway via the enhanced influence and upwelling of high-latitude southern-source waters.

The Influence of the Vietnam Offshore Current and Kuroshio Intrusion on Net Community Production in the Oligotrophic South China Sea in Summer

AbstractIn summer, the Vietnam Offshore Current (VOC) and the Kuroshio intrusion are two important processes inducing considerable environmental fluctuations in the surface water of the South China Sea (SCS). Net community production (NCP) is an important proxy of the biological pump strength and can be estimated based on the dissolved oxygen to argon ratio (O2/Ar) in the mixed layer. To quantify the influence of the VOC and Kuroshio intrusion on NCP in the oligotrophic SCS, we obtained continuous measurements of O2/Ar in the upper mixed layer using membrane inlet mass spectrometry (MIMS) in the northeastern SCS in summer 2017. Average NCP was 2.6 ± 1.5 mmol C m−2 d−1 in Phase 1 (from 13 to 20 July), 5.3 ± 1.7 mmol C m−2 d−1 in Phase 2 (from 20 to 25 July), and 13.9 ± 2.3 mmol C m−2 d−1 in Phase 3 (from 30 July to 9 August). Data from the region influenced by vertical mixing were excluded. A strong correlation between the surface dissolved inorganic nitrogen (DIN) and NCP was obtained, indicating that DIN was an essential factor controlling NCP in the SCS surface water. During our sampling, the Kuroshio intruded into the northeastern SCS in a loop pattern through the Luzon Strait, approximately halving the surface DIN concentration and reducing NCP by 36%. In addition, the VOC drove Vietnam coastal upwelled water to extend northeastward, causing an increase of 140% in the surface DIN concentration and an associated elevation of 160% in NCP in the northeastern SCS.

Deep water pathways in the North Pacific Ocean revealed by Lagrangian particle tracking

Lagrangian particle tracking experiments are conducted to investigate the pathways of deep water in the North Pacific Ocean. The flow field is taken from a state-of-the-art deep circulation simulation. An unprecedented number of particles are tracked to quantify the volume transport and residence time. Half of the North Pacific deep water returns to the Southern Ocean, and its principal pathway is along the western boundary current in the Southwest Pacific Basin in the deep layer. About 30% is exported to the Indian Ocean after upwelling to the shallow layer in the western North Pacific Ocean. The rest is transported to the Arctic Ocean through the Bering Strait or evaporates within the Pacific Ocean. Upwelling of deep water is confined in the western North Pacific Ocean owing to the strong vertical mixing. The mean residence time of deep water in the North Pacific Ocean is estimated to be several hundred years, which is consistent with the observed radiocarbon distribution.

The influence of seasonal calcification depth change on the planktonic foraminiferal stable oxygen isotope signal

Seasonal changes in temperature and habitat migration both leave large imprints on the stable oxygen isotope composition (δ18O) of planktonic foraminiferal tests, but little is known about how much the depth habitat change affects the calcite δ18O on seasonal timescales, potentially introducing numerous errors to the study of seasonality applied individual foraminifera. In this study, two time series sediment trap moorings deployed in the northern South China Sea (SCS) from July 2012 to April 2013 and southern SCS (upwelling area) from April to October 2014 were used to infer seasonal calcification depth variations for four commonly used planktonic foraminiferal species, Globigerinoides ruber, Trilobatus sacculifer, without a sac-like final chamber, Pulleniatina obliquiloculata and Neogloboquadrina dutertrei by analyzing their test δ18O values. The findings demonstrate that planktonic foraminifera deposit their tests shallower in the winter and during cold water upwelling activity, and deeper in the summer, with a changing seasonal depth habitat for all species that tends to follow the isothermals. More importantly, we devised a new approach for quantifying the effect of seasonal calcification depth changes on the δ18O signature of planktonic foraminiferal tests. After applying this method to our trap site and nine other published global sediment trap sites, the results show that foraminiferal seasonal δ18O signal is strongly distorted (amplified or damped) by seasonal variations in their habitat depth, particularly for the species living in low latitude, attributed to the strong vertical temperature gradient relative to the seasonal temperature change at a given water depth. Surface water species that live in the homogeneous mixed layer may be able to provide the most comprehensive seasonality information.

Impacts of Interannual Variations of Chlorophyll on Seasonal Predictions of the Tropical Pacific

In this study, we explored impacts of interannual variations of chlorophyll on seasonal predictions of the tropical Pacific by the SINTEX-F2 dynamical climate prediction system, which is highly skillful at predicting El Niño/Southern Oscillation (ENSO) and other tropical climate phenomena. We conducted twin re-forecast experiments; one system used the observed climatology of chlorophyll to compute the shortwave absorption in the upper ocean, while the other used the observed chlorophyll with year-to-year variations. Although the chlorophyll impacts on predictions of the Niño 3.4 index were limited, improvements are noticed in the predictions of sea surface temperature over the eastern edge of the Western Pacific Warm Pool. This region corresponds to the separation between warm, low-salinity waters of the warm Pool and cold, high-salinity upwelled waters of the Pacific cold tongue in the central-eastern equatorial Pacific. The improvement was very striking in the 2015 case, when a super El Nino occurred.

Climate and ocean circulation in the aftermath of a Marinoan snowball Earth

Abstract. When a snowball Earth deglaciates through a very high atmospheric CO2 concentration, the resulting inflow of freshwater leads to a stably stratified ocean, and the strong greenhouse conditions drive the climate into a very warm state. Here, we use a coupled atmosphere–ocean general circulation model, applying different scenarios for the evolution of atmospheric CO2, to conduct the first simulation of the climate and the three-dimensional ocean circulation in the aftermath of the Marinoan snowball Earth. The simulations show that the strong freshwater stratification breaks up on a timescale of the order of 103 years, mostly independent of the applied CO2 scenario. This is driven by the upwelling of salty waters in high latitudes, mainly the Northern Hemisphere, where a strong circumpolar current dominates the circulation. In the warmest CO2 scenario, the simulated Marinoan supergreenhouse climate reaches a global mean surface temperature of about 30 ∘C under an atmospheric CO2 concentration of 15×103 parts per million by volume, which is a moderate temperature compared to previous estimates. Consequently, the thermal expansion of seawater causes a sea-level rise of only 8 m, with most of it occurring during the first 3000 years. Our results imply that the surface temperatures of that time were potentially not as threatening for early metazoa as previously assumed. Furthermore, the short destratification timescale found in this study implies that Marinoan cap dolostones accumulated during the deglacial period, given that they were deposited under the influence of a freshwater environment.

Modelling the effect of submarine iceberg melting on glacier-adjacent water properties

Abstract. The rate of ocean-driven retreat of Greenland's tidewater glaciers remains highly uncertain in predictions of future sea level rise, in part due to poorly constrained glacier-adjacent water properties. Icebergs and their meltwater contributions are likely important modifiers of fjord water properties, yet their effect is poorly understood. Here, we use a 3-D ocean circulation model, coupled to a submarine iceberg melt module, to investigate the effect of submarine iceberg melting on glacier-adjacent water properties in a range of idealised settings. Submarine iceberg melting can modify glacier-adjacent water properties in three principal ways: (1) substantial cooling and modest freshening in the upper ∼50 m of the water column; (2) warming of Polar Water at intermediate depths due to iceberg melt-induced upwelling of warm Atlantic Water and; (3) warming of the deeper Atlantic Water layer when vertical temperature gradients through this layer are steep (due to vertical mixing of warm water at depth) but cooling of the Atlantic Water layer when vertical temperature gradients are shallow. The overall effect of iceberg melt is to make glacier-adjacent water properties more uniform with depth. When icebergs extend to, or below, the depth of a sill at the fjord mouth, they can cause cooling throughout the entire water column. All of these effects are more pronounced in fjords with higher iceberg concentrations and deeper iceberg keel depths. These iceberg melt-induced changes to glacier-adjacent water properties will reduce rates of glacier submarine melting near the surface, increase them in the Polar Water layer, and cause typically modest impacts in the Atlantic Water layer. These results characterise the important role of submarine iceberg melting in modifying ice sheet-ocean interaction and highlight the need to improve representations of fjord processes in ice sheet scale models.

Baroclinic Control of Southern Ocean Eddy Upwelling Near Topography

AbstractIn the Southern Ocean, mesoscale eddies contribute to the upwelling of deep waters along sloping isopycnals, helping to close the upper branch of the meridional overturning circulation. Eddy energy (EE) is not uniformly distributed along the Antarctic Circumpolar Current (ACC). Instead, “hotspots” of EE that are associated with enhanced eddy‐induced upwelling exist downstream of topographic features. This study shows that, in idealized eddy‐resolved simulations, a topographic feature in the ACC path can enhance and localize eddy‐induced upwelling. However, the upwelling systematically occurs in regions where eddies grow through baroclinic instability, rather than in regions where EE is large. Across a range of parameters, along‐stream eddy growth rate is a more reliable indicator of eddy upwelling than traditional parameterizations such as eddy kinetic energy, eddy potential energy, or isopycnal slope. Ocean eddy parameterizations should consider metrics specific to the growth of baroclinic instability to accurately model eddy upwelling near topography.

Calcium Carbonate Dissolution Triggered by High Productivity During the Last Glacial–Interglacial Interval in the Deep Western South Atlantic

Studies reconstructing surface paleoproductivity and benthic environmental conditions allow us to measure the effectiveness of the biological pump, an important mechanism in the global climate system. In order to assess surface productivity changes and their effect on the seafloor, we studied the sediment core SAT-048A, spanning 43–5 ka, recovered from the continental slope (1,542 m water depth) of the southernmost Brazilian continental margin, deep western South Atlantic. We assessed the sea surface productivity, the organic matter flux to the seafloor, and calcite dissolution effects, based on micropaleontological (benthic and planktonic foraminifers, ostracods), geochemical (benthic δ13C isotopes), and sedimentological data (carbonate and bulk sand content). Superimposed on the induced changes related to the last glacial–interglacial transition, the reconstruction indicates a significant and positive correlation between the paleoproductivity proxies and the summer insolation. From the reconstructed data, it was possible to identify high (low) surface productivity, high (low) organic matter flux to the seafloor, and high (low) dissolution rates of planktonic Foraminifera tests during the glacial (postglacial). Furthermore, within the glacial, enhanced productivity was associated with higher insolation values, explained by increased northeasterly summer winds that promoted meandering and upwelling of the nutrient-rich South Atlantic Central Water. Statistical analyses support the idea that productivity is the main cause for seafloor calcium carbonate dissolution, as opposed to changes in the Atlantic Meridional Overturning Circulation (at least for the 25–4 ka period). Further efforts must be invested in the comprehension and quantification of the total organic matter and biogenic carbonate burial during time intervals with an enhanced biological pump, aiming to better understand their individual roles.

Internal seiches as drivers of fish depth use in lakes

AbstractInland temperate lakes undergo various physical processes, such as thermal stratification, that dictate the spatial availability of suitable temperature and dissolved oxygen conditions. Here, we use intensive limnological monitoring and acoustic telemetry transmitters implanted in wild fish to document the magnitude and frequency of thermocline deflection events (i.e., wind driven internal seiches that lead to upwelling of hypoxic hypolimnetic water) and their influence on freshwater fish depth use in a coastal embayment of Lake Ontario. The embayment experienced around 100 internal seiche events during a 3‐month period and tracking of walleye (Stizostedion vitreus) vertical positions in the water column showed clear trends of avoidance of low dissolved oxygen. Quantile regression showed a significant correlation between walleye vertical position and the depth of the 3, 4, and 5 mg L−1 oxyclines, with the 3 mg L−1 oxycline having the largest effect. Upwellings of the hypoxic hypolimnion forced walleye to use the water column above these fluctuating oxyclines (5th percentile, p < 0.001), with 94.2% of detections occurring at depths above the 3 mg L−1 oxycline. Understanding how fish respond to upwelling events (both temperature and oxygen) is important for fisheries assessment, management, and habitat restoration planning as there is clear avoidance of suboptimal oxygen conditions and sampling in the overcrowded fringes of these low‐oxygen zones could artificially inflate population estimates.